Size reduction of metal particles



3,090,567 SIZE REDUCTION OF METAL PARTECLES Robert J. chafer, 150 MeadowDrive, Berea, Ohio, and Max Quatinetz, 5821 Woodman Court, Par-ma, OhioNo Drawing. Filed Sept. 19, 1%0, Ser. No. 56,647 4 Claims. (Cl. 24l22)This invention relates to a novel method of reducing the size of metalparticles and, more particularly, it relates to a method for reducingthe particle size of rn-etads below about one micron.

The search for methods capable of producing very fine metal powders hasbecome increasingly important as the use of metal powders in powdermetallurgy techniques and other applications requiring fine powders hasmultiplied. While fine metal powders have been produced by nucleationand growth and also mechanical comminution means, the use of nucleationand growth methods is more widespread because of the difliculties inmechanically reducing metal particles to a suflicient degree offineness. In some instances where comrninution means have been employed,there is no reduction in the particle size, but rather the particlestend to increase in size due to agglomeration of the very fineparticles. However, although metal powders in the submicron range may beproduced, the methods employed have not been considered completelysatisfactory because of the high cost and lack of flexibility generallyinvolved in such methods.

In accordance with the present invention, it has been found that theparticle size of a wide variet of metals may be successfully reducedbelow about one micron by mechanical means. Not only does the method ofthe invention provide for the reduction of metal particles by mechanicalmeans, but also the method employs conventional grinding and millingequipment to achieve the desired particle size reduction. Moreover, theinvention provides an economically feasible method for producingcommercial quantities of such relatively fine metal powders.

The reduction of metal particles to a size below about one micron isachieved in accordance with the present invention by a method whichcomprises mixing the metal particles with an inorganic salt and asufficient quantity of a liquid diluent to form a flowable slurry. Theslurry is milled to reduce the size of the particles after which thefinely powdered metal may be separated from the salt and diluent. Theinorganic salt employed consists essentially of a compound formed fromat least one polyvalent ion and is composed of more than two chemicalelements. Advantageously, the polyvalent ion is trivalent ortetravalent, and preferably, the anion portion of the compound containsat least two chemical elements.

Among the salts with polyvalent ions containing more than two chemicalelements which are suitable for em ployment in the method of the presentinvention are the salts of metals such as sodium potassium, aluminum,copper, and salts of non-metfls such as ammonia. The salts may be formedfrom acids such as nitric acid, sulfuric acid, phosphoric acid, chromicacid, and the like. Advantageously, the salts are soluble in a washingliquid to facilitate the separation of the salt from the metal powderafter the grinding operation.

While it is essential that the salt employed in the method of thepresent invention contain a polyvalent ion, it is not necessary thatboth the cation and anion portions of the compound be polyvalent. Forexample, the salt may be a sodium salt of pyrophosphoric acid and, thus,contain the monovalent sodium ion with the tetravalent pyrophosphateradical. Likewise, a salt such as aluminum nitrate might be employed inwhich the aluminum is trivalent While the nitrate radical is monovalent.However, salts such as aluminum sulfate, in which both ions of thecompound are polyvalent, also may be employed.

The salts employed in the method of the present invention may beanhydrous or hydrated salts although if hydrated salts are employed, thequantity thereof may be increased to provide a concentrationsubstantially the same as if an anhydrous salt were employed.

The liquid diluent employed in combination with the inorganic salt maybe a conventional solvent or diluent. Although the liquid may be wateror an aqueous solution, organic diluents are preferred. Advantageously,the diluent may be a hydrocarbon such as heptane, octane, etc.; analcohol such as methyl or ethyl alcohol, etc.; a cyclic compound such ascyclohexa-ne, etc.; or a chlorinated hydrocarbon such as methylenechloride, etc.

The proportions of the inorganic salt and the diluent may vary over awide range. While the specific proportions will depend in part upon theparticular materials employed, the minimum proportion of the diluentshould be sufficient to maintain the slurry in a flowable state duringcomminu-tion while the salt Will generally comprise at least about 5 or10% by weight of the metal particles. The maximum proportions are notcritical since the employment of large quantities of the salt or diluentwill generally only result in a lowering of the grinding efficiency,i.e., a reduction in the quantity of metal powder produced in a givenperiod of time or with a given amount of power. Thus, the proportion ofthe salt may vary from about one-tenth up to two or three or more timesthe weight of the metal particles. Generally, the diluent will comprisesomewhere in the range of about 50% to 300% by weight of the totalsolids incorporated in the slurry and preferably between about and 200%of such solids.

The method of the present invention may be employed to reduce the sizeof a wide variety of ductile, brittle andrefractory metals. For example,the method of the invention may be employed to reduce the particle sizeof metals such as copper, silver, nickel, chromium, iron, tungsten,molybdenum, and columbium as well as a great many other metals andalloys.

The following examples describe in greater detail the production ofpowdered metals in accordance with the method of the present invention.All of the experiments except Example V=III were conducted in the sameapparatus employing the following general procedure. A powdered metalslurry containing the inorganic salt and the organic liquid diluent wasmilled by placing the slurry in a ball mill jar having an insidediameter of about 4% inches and an inside height of about 5 inches. Theca pacity of the jar was approximately three pints. The jar wasconstructed of an austenitic stainless steel and had three internal ribsrunning the length of the jar. These ribs were inch square incross-section and were spaced equidistantly around the insidecircumference of the jar.

The balls used were /2 inch diameter type 410 stainless steel balls. Asufficient number of balls to fill the jar approximately half full andweighing approximately 3000 grams was employed. The jar containing theslurry and the balls was placed on a rolling mill having a triple set ofrolls four feet in length which could accommodate 21 jarssimultaneously. The jar was rotated at 48 rpm. for several days as setforth below, after which the slurry was removed from the jar. maximumtime that the jars were rotated in the ball mill was 15 days, but thisis not tobe considered a limitation of the invention as to the time ofgrinding.

The slurries obtained in each of the examples were allowed to stand tosettle the metal powder and the salt. The liquid diluent was thendecanted OE and the solids Arbitrarily in the examples, the

washed with hot (water to dissolve the inorganic salts. The

washing operation was repeated ten times. Each time, the metal powderwas placed in a beaker filled with water, the mixture was stirredthoroughly after which the powder was allowed to settle and theliquid'was then decanted ofi. After ten washing cycles, the metal powderwas mixed with 190 proof ethyl alcohol and stirred. The resultingsuspension was then filtered through a Buchner funnel and the filtercake dried in air at room temperature. The dried filter cake was crushedand then stirred for approximately one minute in an Osterizer. 7 Afterthis step, samples of each powder were tested to determine the particlesize using known methods.

Example I In this example, a number of different inorganic salts wereemployed with the same metal and diluent and following the proceduredescribed above.

The metal employed was a nickel powder having an average particle sizeof 2.55 microns sold by the International Nickel Company as Incocarbonyl grade B nickel powder. 210 grams of the nickel powder werecombined with 300 milliliters of 200 proof ethyl alcohol and'70 grams ofsalt. When hydrated salts were employed, the amount of salt used wasbased on the amount calculated to provide 70 grams of anhydrousmaterial;

The following table lists for each salt the particle size obtained afterthe specified number of days.

Example 11' The procedure of this example was the same as Example Iexcept that methylene chloride was substituted for the alcohol diluentwith the inorganic salts listed below:

Grinding Average Compounds Formula time particle (days) size (microns)Sodium pyrophosphate a P201 15 0.23 Aluminum nitrate 2 0. 35

Example 111 The procedure employed in this example was the same asExample I except for the following: 600 milliliters of ethyl alcoholwere employed in each mixture. The total weight of the metal powder andsalt combined was 400 grams with the proportions being varied in eachrun. Ammonium sul-fite was employed as the salt in each run and themetal powder was the nickel powder of Example I. The quantity ofstainless steel balls was increased to 3120 grams from the 3000 gramsemployed in Example I. The following table gives the results for'each ofthe runs:

Metal Salt Grinding Average Run powder (grams) time (days) particle size(grams) (microns) Example IV I The procedure of this example was thesame as Example I except that potassium ferricyanide was employed as thesalt in each of the runs and the proportions of the salt and the nickelmetal powder were carried in each run.

Metal Salt Grinding Average Run powder (grams) time (days) particle size(grams) (microns) Example V The procedure employed in this example wasthe same as that of Example I except that different size nickel powderswere employed as the starting materials. The salt used was potassiumferricyanide. The following table gives the results of experimentsemploying powders having diiferent original particle sizes:

Original Final average Grinding average Source of nickel powdersparticle time particle size (ini- (days) size (microns) crons) 23 8 0.2817 15 0. 26 9. 3 15 0. 25 4. 8 8 0. 42 Sherritt Gordon C0,, FF 2.1 8 0.31

Example VI The procedures employed in this example were the same asExample V except that powders of various metals other than nickel wereemployed. The following table gives the results with the listed powders:

Example VII The procedure of this example was the same as Example Iexcept for the following: I

1750 grams of the nickel powder, 250 grams of ammonium sulfite, and 3000grams of ethyl alcohol were employed. The mill jar was of a somewhatlarger capacity than those employed in the foregoing examples having acapacity of approximately one gallon each. 15.6 kilograms of stainlesssteel balls were employed in the mill jar. After grinding for ten days,the average particle size was reduced to 0.45 micron.

Example VIII 150 grams of the nickel powder employed in Example I weremixed with 50 grams of ammonium suliite and 200 milliliters of 200 proofethyl alcohol. This mixture was then combined with 2000 grams of /8 inchdiameter type 410 stainless steel balls in a Research Model No. 01Szegvari Attritor mill manufactured by Union Process Company of Akron,Ohio. The grinding tank capacity of the mill was approximately 750milliliters. The agitator was operated at a speed of about r.p.m. Afterone day, the average particle size was reduced to 0.22 micron.Continuing the operation for an additional 16 hours reduced the averageparticle size to 0.15 micron.

As shown by the foregoing description and specific examples, theemployment of certain inorganic salts and liquid diluents in thegrinding of metal powders results in the production of especially finemetal powders having average particle sizes of less than about 1 micronand in many cases less than 0.5 micron. Particularly advantageousresults are achieved when the inorganic salt is a water-soluble salt andthe diluent is an organic liquid.

The method of the invention not only provides for a reduction in theparticle size of metals but achieves this reduction more economicallythan was possible with methods heretofore employed. Since the particlesize reduction may be carried out in conventional grinding and millingapparatus, the method of the invention permits the production of suchfine metal powders in commercial quantities.

It is apparent from the above description that various modifications inthe specific materials and procedures described may be made within thescope of the invention. Therefore, the invention is not intended to belimited to the particular materials and procedures described in detailherein except as may be required by the appended claims.

What is claimed is:

1. A method of reducing metal particles to a size below about one micronwhich comprises mixing said metal particles, an inorganic salt and asufificient quantity of a liquid diluent to form a fiowable slurry, andmilling said slurry to reduce the size of said metal particles, saidsalt consisting essentially of a compound formed from at least onepolyvalent ion and being composed of more than two chemical elements andsaid salt comprising between about 5% and 300% by weight of said metalparticles, and said diluent being chemically inert to said metalparticles and said salt and comprising between about 50% and 300% byweight of the total metal particles and salt.

2. A method of reducing metal particles to a size below about one micronwhich comprises mixing said metal particles, an inorganic salt and asuflicient quantity of a liquid diluent to form a flowable slurry, andmilling said slurry to reduce the size of said metal particles, saidsalt consisting essentially of a water-soluble compound formed from atleast one polyvalent ion and being composed of more than two chemicalelements and said salt comprising between about 5% and 300% by weight ofsaid metal particles, and said diluent being chemically inert to saidmetal particles and said salt and comprising between about and 300% byweight of the total metal particles and salt.

3. A method of reducing metal particles to a size below about one micronwhich comprises mixing said metal particles, an inorganic salt and asufficient quantity of a liquid diluent to form a fiowable slurry, andmilling said slurry to reduce the size of said metal particles, saidsalt consisting essentially of a compound formed from at least onepolyvalent ion and being composed of more than two chemical elements andsaid salt comprising between about 10% and 300% by weight of said metalparticles, and said diluent being chemically inert to said metalparticles and said salt and comprising between about 50% and 300% byweight of the total metal particles and salt.

4. A method of reducing metal particles to a size below about one micronwhich comprises mixing said metal particles, an inorganic salt and asufficient quantity of an organic liquid diluent to form a flowableslurry, milling said slurry to reduce the size of said metal particles,and separating said reduced size metal particles from said salt and saiddiluent, said salt consisting essentially of a watersoluble compoundformed from at least one polyvalent ion and an anion composed of atleast two chemical elements and said salt comprising between about 10%and 300% by weight of said metal particles, and said diluent beingchemically inert to said metal particles and said salt and comprisingbetween about and 200% by weight of the total metal particles and salt.

Taggart: Pages 22-67, Handbook of Mineral Dressing, Wiley and Sons, NewYork, copyright 1945.

1. A METHOD OF REDUCING METAL PARTICLES TO A SIZE BELOW ABOUT ONE MICRONWHICH COMPRISES MIXING SAID METAL PARTICLES, AN INORGANIC SALT AND ASUFFICIENT QUANTITY OF A LIQUID DILUENT TO FORM A FLOWABLE SLURRY, ANDMILLING SAID SLURRY TO REDUCE THE SIZE OF SAID METAL PARTICLES, SAIDSALT CONSISTING ESSENTIALLY OF A COMPOUND FORMED FROM AT LEAST ONEPOLYVALENT ION AND BEING COMPOSED OF MORE THAN TWO CHEMICAL ELEMENTS ANDSAID SALT COMPRISING BETWEEN ABOUT 5% AND 300% BY WEIGHT OF SAID METALPARTICLES, AND SAID DILUENT BEING CHEMICALLY INERT TO SAID METALPARTICLES AND SAID SALT AND COMPRISING BETWEEN ABOUT 50% AND 300% BYWEIGHT OF THE TOTAL METAL PARTICLES AND SALT.